Thermal switchable composition and imaging member containing complex oxonol IR dye and methods of imaging and printing

ABSTRACT

An imaging member, such as a negative-working printing plate or on-press cylinder, can be prepared with a hydrophilic imaging layer comprised of a heat-sensitive hydrophilic charged polymer (ionomer) and an infrared radiation sensitive negatively-charged oxonol dye that has a λ max  of greater than 700 nm. The heat-sensitive polymer and IR dye can be formulated in water or water-miscible solvents to provide highly thermal sensitive imaging compositions. In the imaging member, the polymer reacts to provide increased hydrophobicity in areas exposed to energy that provides or generates heat. For example, heat can be supplied by laser irradiation in the IR region of the electromagnetic spectrum. The heat-sensitive polymer is considered “switchable” in response to heat, and provides a lithographic image without conventional alkaline processing.

FIELD OF THE INVENTION

[0001] This invention relates in general to thermal imaging compositionsand to imaging members (particularly lithographic printing plates)prepared therefrom. The invention also relates to a method of imagingsuch imaging members, and to a method of printing using them.

BACKGROUND OF THE INVENTION

[0002] The art of lithographic printing is based upon the immiscibilityof oil and water, wherein an oily material or ink is preferentiallyretained by an imaged area and the water or fountain solution ispreferentially retained by the non-imaged areas. When a suitablyprepared surface is moistened with water and ink is then applied, thebackground or non-imaged areas retain the water and repel the ink whilethe imaged areas accept the ink and repel the water. The ink is thentransferred to the surface of a suitable substrate, such as cloth, paperor metal, thereby reproducing the image.

[0003] Very common lithographic printing plates include a metal orpolymer support having thereon an imaging layer sensitive to visible orUV light. Both positive- and negative-working printing plates can beprepared in this fashion. Upon exposure, and perhaps post-exposureheating, either imaged or non-imaged areas are removed using wetprocessing chemistries.

[0004] Thermally sensitive printing plates are becoming more common.Examples of such plates are described in U.S. Pat. No. 5,372,915 (Haleyet al.). They include an imaging layer comprising a mixture ofdissolvable polymers and an infrared radiation-absorbing compound. Whilethese plates can be imaged using lasers and digital information, theyrequire wet processing using alkaline developer solutions.

[0005] It has been recognized that a lithographic printing plate couldbe created by ablating an IR absorbing layer. For example, Canadian1,050,805 (Eames) discloses a dry planographic printing plate comprisingan ink receptive substrate, an overlying silicone rubber layer, and aninterposed layer comprised of laser energy absorbing particles (such ascarbon particles) in a self-oxidizing binder (such as nitrocellulose).Such plates were exposed to focused near IR radiation with a Nd⁺⁺YAGlaser. The absorbing layer converted the infrared energy to heat thuspartially loosening, vaporizing or ablating the absorber layer and theoverlying silicone rubber.

[0006] While the noted printing plates used for digital, processlessprinting have a number of advantages over the more conventionalphotosensitive printing plates, there are a number of disadvantages withtheir use. The process of ablation creates debris and vaporizedmaterials that must be collected. The laser power required for ablationcan be considerably high, and the components of such printing plates maybe expensive, difficult to coat, or unacceptable for resulting printingquality. Such plates generally require at least two coated layers on asupport.

[0007] Thermally switchable polymers have been described for use asimaging materials in printing plates. By “switchable” is meant that thepolymer is rendered from hydrophobic to relatively more hydrophilic or,conversely from hydrophilic to relatively more hydrophobic, uponexposure to heat.

[0008] U.S. Pat. No. 4,034,183 (Uhlig) describes the use of high-poweredlasers to convert hydrophilic surface layers to hydrophobic surfaces. Asimilar process is described for converting polyamic acids intopolyimides in U.S. Pat. No. 4,081,572 (Pacansky). The use ofhigh-powered lasers is undesirable in the industry because of their highelectrical power requirements and because of their need for cooling andfrequent maintenance.

[0009] U.S. Pat. No. 4,634,659 (Esumi et al.) describes imagewiseirradiating hydrophobic polymer coatings to render exposed regions morehydrophilic in nature. While this concept was one of the earlyapplications of converting surface characteristics in printing plates,it has the disadvantages of requiring long UV light exposure times (upto 60 minutes), and the plate's use is in a positive-working mode only.

[0010] U.S. Pat. No. 4,405,705 (Etoh et al.) and U.S. Pat. No. 4,548,893(Lee et al.) describe amine-containing polymers for photosensitivematerials used in non-thermal processes. Thermal processes usingpolyamic acids and vinyl polymers with pendant quaternary ammoniumgroups are described in U.S. Pat. No. 4,693,958 (Schwartz et al.). U.S.Pat. No. 5,512,418 (Ma) describes the use of polymers having cationicquaternary ammonium groups that are heat-sensitive. However, thematerials described in this art require wet processing after imaging.

[0011] WO 92/09934 (Vogel et al.) describes photosensitive compositionscontaining a photoacid generator and a polymer with acid labiletetrahydropyranyl or activated ester groups. However, imaging of thesecompositions converts the imaged areas from hydrophobic to hydrophilicin nature.

[0012] In addition, EP-A 0 652 483 (Ellis et al.) describes lithographicprinting plates imageable using infrared radiation (“IR”) lasers, andwhich do not require wet processing. These plates comprise an imaginglayer that becomes more hydrophilic upon imagewise exposure to heat.This coating contains a polymer having pendant groups (such as t-alkylcarboxylates) that are capable of reacting under heat or acid to formmore polar, hydrophilic groups. Imaging such compositions converts theimaged areas from hydrophobic to relatively more hydrophilic in nature,and thus requires imaging the background of the plate, which isgenerally a larger area. This can be a problem when imaging to the edgeof the printing plate is desired.

[0013] U.S. Pat. No. 5,985,514 (Zheng et al.) is directed to processlessdirect write printing plates that include an imaging layer containingheat sensitive polymers. The polymer coatings are sensitized to infraredradiation by the incorporation of an infrared absorbing material such asan organic dye or a fine dispersion of carbon black. Upon exposure to ahigh intensity infrared laser, light absorbed by the organic dye orcarbon black is converted to heat, thereby promoting a physical and/orchemical change in the polymer (usually a change in hydrophilicity orhydrophobicity). The resulting printing plates can be used onconventional printing presses to provide, for example, negative images.Such printing plates have utility in the evolving “computer-to-plate” or“direct-write” printing market.

[0014] Other imaging members having “switchable” imaging layers aredescribed in U.S. Pat. No. 6,146,812 (Leon et al.) wherein switchingoccurs rapidly yet the heat-sensitive polymers have improved shelf lifestability.

[0015] Some of the heat-sensitive polymers described in the copendingapplications, particularly the polymers containing organoonium or othercharged groups, have a tendency to undergo physical interactions orchemical reactions with the organic dye or carbon black, thuscompromising the effectiveness of both polymers and heat-absorbingmaterials.

[0016] Organic dye salts, by nature, are often partially soluble inwater or alcoholic coating solvents and are thus preferred as IR dyesensitizers. However, many such salts have been found to be unacceptablebecause of insufficient solubility, because they react with the chargedpolymer to form hydrophobic products that can result in scummed or tonedimages, or because they offer insufficient thermal sensitization inimaging members. In particular, there is a need to have IR dyesensitizers that are compatible with thiosulfate polymers, such as thosedescribed in U.S. Pat. No. 5,985,514 (noted above).

[0017] Other imaging members comprise cationic heat-sensitive ionomersthat are used in combination with IR-sensitive dyes or carbon.Representative cationic ionomers are described for example in U.S. Pat.No. 6,190,830 (Leon et al.) and U.S. Pat. No. 6,190,831 (Leon et al.).

[0018] Improved thermally sensitive compositions and imaging members arealso described in GB 2,358,710 (DoMinh et al.). These compositionscomprise IR sensitive oxonol dyes that are described in U.S. Pat. No.6,248,886 (Williams et al.) and U.S. Pat. No. 6,248,893 (Williams etal.).

[0019] There is a need for direct-write lithographic imaging membersthat contain IR sensitive compounds that have improved compatibilitywith various charged thermally sensitive polymers.

SUMMARY OF THE INVENTION

[0020] The problems noted above are overcome with a heat-sensitivecomposition comprising:

[0021] a) a hydrophilic heat-sensitive ionomer,

[0022] b) water or a water-miscible organic solvent, and

[0023] c) an infrared radiation sensitive negatively-charged oxonol dyethat has a λ_(max) greater than 700 nm as measured in water or awater-miscible organic solvent,

[0024] the negatively-charged oxonol dye being represented by thefollowing Structure I:

[0025] wherein R′ is a substituted or unsubstituted alkyl group,substituted or unsubstituted cycloalkyl group, substituted orunsubstituted heterocyclic group, or substituted or unsubstitutedcarbocyclic aromatic group, R₁′ and R₂′ are independently substituted orunsubstituted heterocyclic or carbocyclic aromatic groups, and M⁺ is amonovalent cation.

[0026] This invention also provides a negative-working imaging membercomprising a support and having disposed thereon a hydrophilic imaginglayer that is prepared from the heat-sensitive composition describedabove.

[0027] Still further, this invention includes a method of imagingcomprising the steps of:

[0028] A) providing the negative-working imaging member described above,and

[0029] B) imagewise exposing the imaging member to provide exposed andunexposed areas in the imaging layer of the imaging member, whereby theexposed areas are rendered more hydrophobic than the unexposed areas byheat provided by the imagewise exposure.

[0030] Still again, a method of printing comprises the steps of carryingout steps A and B noted above, and additionally:

[0031] C) contacting the imagewise exposed imaging member with alithographic printing ink, and imagewise transferring that printing inkfrom the imaging member to a receiving material.

[0032] As used herein, the term “ionomer” refers to a positively ornegatively charged polymer having at least 15 mol % of the recurringunits negatively charged.

[0033] The imaging members of this invention have a number ofadvantages, and provide solutions to the problems recognized in previousprinting plates. Specifically, the problems and concerns associated withablation imaging (that is, imagewise removal of a surface layer) areavoided because the hydrophilicity of the imaging layer is changedimagewise by “switching” (preferably, irreversibly) exposed areas of itsprinting surface to be less hydrophilic (that is, become morehydrophobic when heated). Thus, the imaging layer stays intact duringand after imaging (that is, no ablation occurs). These advantages areachieved by using a hydrophilic heat-sensitive polymer (ionomer) havingrecurring charged groups within the polymer backbone or chemicallyattached thereto. Such polymers and groups are described in more detailbelow. The polymers used in the imaging layer are readily prepared usingprocedures described herein, and the imaging members of this inventionare simple to make and use without the need for post-imaging wetprocessing. The resulting printing members formed from the imagingmembers of this invention are negative working in nature. Moreover,conventional alkaline development is not necessary with the imagingmembers of this invention.

[0034] Charged polymers that are used in the practice of this inventionare typically coated out of water and methanol, and other water-misciblesolvents that will readily dissolve these water-soluble polymeric salts.

[0035] The “complex” oxonol infrared radiation-sensitive dyes (“IR dyes”herein) used in this invention are negatively charged IR sensitizers forthermal imaging members because they can be selected to have maximumabsorption at the operating wavelength of a laser platesetter (generally700 nm or more). Moreover, they can be coated in a dissolved (that ismolecularly dispersed) state, providing for maximized utilization ofenergy as well as maximized image resolution capability. Theheat-sensitive compositions of this invention provide increasedphotospeed at reduced IR dye coverage and release minimal gaseouseffluents. Furthermore, we have not observed adverse effects from aninteraction of ionomers polymers (particularly thiosulfate polymers) andthe negatively charged oxonol IR dyes useful in the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0036] The imaging members of this invention comprise a support and oneor more layers disposed thereon that include a dried heat-sensitivecomposition. The support can be any self-supporting material includingpolymeric films, glass, ceramics, cellulosic materials (includingpapers), metals or stiff papers, or a lamination of any of thesematerials. The thickness of the support can be varied. In mostapplications, the thickness should be sufficient to sustain the wearfrom printing and thin enough to wrap around a printing form. Apreferred embodiment uses a polyester support prepared from, forexample, polyethylene terephthalate or polyethylene naphthalate, andhaving a thickness of from about 100 to about 310 μm. Another preferredembodiment uses aluminum sheets having a thickness of from about 100 toabout 600 μm. The support should resist dimensional change underconditions of use.

[0037] The support may also be a cylindrical support that includesprinting cylinders on press as well as printing sleeves that are fittedover printing cylinders. The use of such supports to provide cylindricalimaging members is described in U.S. Pat. No. 5,713,287 (Gelbart). Theheat-sensitive polymer composition can be coated or sprayed directlyonto the cylindrical surface that is an integral part of the printingpress.

[0038] The support may be coated with one or more “subbing” layers toimprove adhesion of the final assemblage. Examples of subbing layermaterials include, but are not limited to, gelatin and other naturallyoccurring and synthetic hydrophilic colloids and vinyl polymers (such asvinylidene chloride copolymers) that are known for such purposes in thephotographic industry, vinylphosphonic acid polymers, sol gel materialssuch as those prepared from alkoxysilanes (includingglycidoxypropyltriethoxysilane and aminopropyltriethoxysilane), epoxyfunctional polymers, and various ceramics.

[0039] The backside of the support may be coated with antistatic agentsand/or slipping layers or matte layers to improve handling and “feel” ofthe imaging member.

[0040] The imaging members, however, preferably have only one layer onthe support, that is a heat-sensitive surface layer that is required forimaging. This hydrophilic layer is prepared from a heat-sensitivecomposition of this invention and includes one or more heat-sensitiveionomers and one or more negatively charged oxonol IR dyes as aphotothermal conversion material (both described below). Because of theparticular polymer(s) used in the imaging layer, the exposed (imaged)areas of the layer are rendered more hydrophobic in nature. Theunexposed areas remain hydrophilic in nature.

[0041] Thus, in the heat-sensitive imaging layer of the imaging member,only the one or more ionomers and one or more negatively charged oxonolIR dyes are essential for imaging. The charged ionomers generally arecomprised of recurring units, of which at least 15 mol % include anionicgroups. Preferably, at least 20 mol % of the recurring groups includeanionic groups. Thus each of these ionomers has a net positive ornegative charge provided by these anionic groups.

[0042] Representative charged ionomers useful in the practice of thisinvention can be in described in any of three broad classes ofmaterials:

[0043] I) crosslinked or uncrosslinked vinyl polymers comprisingrecurring units comprising positively charged, pendant N-alkylatedaromatic heterocyclic groups,

[0044] II) crosslinked or uncrosslinked polymers comprising recurringorganoonium groups, and

[0045] III) polymers comprising a pendant thiosulfate (Bunte salt)group.

[0046] Each class of polymers is described in turn. The imaging layercan include mixtures of polymers from each class, or a mixture of one ormore polymers of two or more classes. In addition, the imaging layer caninclude one or more ionomers that do not belong in any of these classesof polymers. The Class III polymers are preferred.

[0047] Class I Polymers:

[0048] The Class I polymers generally have a molecular weight of atleast 1000 and can be any of a wide variety of hydrophilic vinylhomopolymers and copolymers having the requisite positively chargedgroups. They are prepared from ethylenically unsaturated polymerizablemonomers using any conventional polymerization technique. Preferably,the polymers are copolymers prepared from two or more ethylenicallyunsaturated polymerizable monomers, at least one of which contains thedesired pendant positively charged group, and another monomer that iscapable of providing other properties, such as crosslinking sites andpossibly adhesion to the support. Procedures and reactants needed toprepare these polymers are well known. With the additional teachingprovided herein, the known polymer reactants and conditions can bemodified by a skilled artisan to attach a suitable cationic group.

[0049] The presence of a cationic group apparently provides orfacilitates the “switching” of the imaging layer from hydrophilic tohydrophobic in the areas that have been exposed to heat in some manner,when the cationic group reacts with its counterion. The net result isthe loss of charge. Such reactions are more easily accomplished when theanion is more nucleophilic and/or more basic. For example, an acetateanion is typically more reactive than a chloride anion. By varying thechemical nature of the anion, the reactivity of the heat-sensitivepolymer can be modified to provide optimal image resolution for a givenset of conditions (for example, laser hardware and power, and printingpress needs) balanced with sufficient ambient shelf life. Useful anionsinclude the halides, carboxylates, sulfates, borates and sulfonates.Representative anions include, but are not limited to, chloride,bromide, fluoride, acetate, tetrafluoroborate, formate, sulfate,p-toluenesulfonate, and others readily apparent to one skilled in theart. The halides and carboxylates are preferred.

[0050] The aromatic cationic group is present in sufficient recurringunits of the polymer so that the heat-activated reaction described abovecan provide desired hydrophobicity of the imaged printing layer. Thegroups can be attached along a principal backbone of the polymer, or toone or more branches of a polymeric network, or both. The aromaticgroups generally comprise 5 to 10 carbon, nitrogen, sulfur or oxygenatoms in the ring (at least one being a positively charged nitrogenatom), to which is attached a branched or unbranched, substituted orunsubstituted alkyl group. Thus, the recurring units containing thearomatic heterocyclic group can be represented by the Structure II:

[0051] In this structure, R₁ is a branched or unbranched, substituted orunsubstituted alkyl group having from 1 to 12 carbon atoms (such asmethyl, ethyl, n-propyl, isopropyl, t-butyl, hexyl, methoxymethyl,benzyl, neopentyl, and dodecyl). Preferably, R₁ is a substituted orunsubstituted, branched or unbranched alkyl group having from 1 to 6carbon atoms, and most preferably, it is substituted or unsubstitutedmethyl group.

[0052] R₂ can be a substituted or unsubstituted alkyl group (as definedabove, and additionally a cyanoalkyl group, a hydroxyalkyl group oralkoxyalkyl group), substituted or unsubstituted alkoxy having 1 to 6carbon atoms (such as methoxy, ethoxy, isopropoxy, oxymethylmethoxy,n-propoxy and butoxy), a substituted or unsubstituted aryl group having6 to 14 carbon atoms in the ring (such as phenyl, naphthyl, anthryl,p-methoxyphenyl, xylyl, and alkoxycarbonylphenyl), halo (such as chloroand bromo), a substituted or unsubstituted cycloalkyl group having 5 to8 carbon atoms in the ring (such as cyclopentyl, cyclohexyl and4-methylcyclohexyl), or a substituted or unsubstituted heterocyclicgroup having 5 to 8 atoms in the ring including at least one nitrogen,sulfur or oxygen atom in the ring (such as pyridyl, pyridinyl,tetrahydrofuranyl and tetrahydropyranyl). Preferably, R₂ is substitutedor unsubstituted methyl or ethyl group.

[0053] Z″ represents the carbon and any additional nitrogen, oxygen, orsulfur atoms necessary to complete the 5- to 10-membered aromaticN-heterocyclic ring that is attached to the polymeric backbone. Thus,the ring can include two or more nitrogen atoms in the ring (forexample, N-alkylated diazinium or imidazolium groups), or N-alkylatednitrogen-containing fused ring systems including, but not limited to,pyridinium, quinolinium, isoquinolinium acridinium, phenanthradinium andothers readily apparent to one skilled in the art.

[0054] W⁻ is a suitable anion as described above. Most preferably it isacetate or chloride.

[0055] Also in Structure II, n is defined as 0 to 6, and is preferably 0or 1. Most preferably, n is 0.

[0056] The aromatic heterocyclic ring can be attached to the polymericbackbone at any position on the ring. Preferably, there are 5 or 6 atomsin the ring, one or two of which are nitrogen. Thus, the N-alkylatednitrogen containing aromatic group is preferably imidazolium orpyridinium and most preferably it is imidazolium.

[0057] The recurring units containing the cationic aromatic heterocyclecan be provided by reacting a precursor polymer containing unalkylatednitrogen containing heterocyclic units with an appropriate alkylatingagent (such as alkyl sulfonate esters, alkyl halides and other materialsreadily apparent to one skilled in the art) using known procedures andconditions.

[0058] Preferred Class I polymers can be represented by the followingStructure III that represents random recurring units derived from one ormore monomers as described below:

[0059] wherein X represents recurring units to which the N-alkylatednitrogen containing aromatic heterocyclic groups (represented by HET⁺)are attached, Y represents recurring units derived from ethylenicallyunsaturated polymerizable monomers that may provide active sites forcrosslinking using any of various crosslinking mechanisms (describedbelow), W⁻ is a suitable anion as described above, and Z representsrecurring units derived from any additional ethylenically unsaturatedpolymerizable monomers. The various repeating units are present insuitable amounts, as represented by x being from about 20 to 100 mol %,y being from about 0 to about 20 mol %, and z being from 0 to 80 mol %.Preferably, x is from about 30 to about 98 mol %, y is from about 2 toabout 10 mol % and z is from 0 to about 68 mol %.

[0060] Crosslinking of the polymers can be provided in a number of ways.There are numerous monomers and methods for crosslinking that arefamiliar to one skilled in the art. Some representative crosslinkingstrategies include, but are not necessarily limited to:

[0061] a) reacting an amine or carboxylic acid or other Lewis basicunits with di-epoxide crosslinkers,

[0062] b) reacting an epoxide units within the polymer withdi-functional amines, carboxylic acids, or other di-functional Lewisbasic unit,

[0063] c) irradiative or radical-initiated crosslinking of doublebond-containing units such as acrylates, methacrylates, cinnamates, orvinyl groups,

[0064] d) reacting a multivalent metal salts with ligating groups withinthe polymer (the reaction of zinc salts with carboxylic acid-containingpolymers is an example),

[0065] e) using crosslinkable monomers that react via the Knoevenagelcondensation reaction, such as (2-acetoacetoxy)ethyl acrylate andmethacrylate,

[0066] f) reacting an amine, thiol, or carboxylic acid groups with adivinyl compound (such as bis (vinylsulfonyl) methane) via a Michaeladdition reaction,

[0067] g) reacting a carboxylic acid units with crosslinkers havingmultiple aziridine units,

[0068] h) reacting a crosslinkers having multiple isocyanate units withamines, thiols, or alcohols within the polymer,

[0069] i) mechanisms involving the formation of interchain sol-gellinkages [such as the use of the 3-(trimethoxysilyl) propylmethacrylatemonomer],

[0070] j) oxidative crosslinking using an added radical initiator (suchas a peroxide or hydroperoxide),

[0071] k) autooxidative crosslinking, such as employed by alkyd resins,

[0072] l) sulfur vulcanization, and

[0073] m) processes involving ionizing radiation.

[0074] Monomers having crosslinkable groups or active crosslinkablesites (or groups that can serve as attachment points for crosslinkingadditives, such as epoxides) can be copolymerized with the othermonomers noted above. Such monomers include, but are not limited to,3-(trimethoxysilyl)propyl acrylate or methacrylate, cinnamoyl acrylateor methacrylate, N-methoxymethyl methacrylamide, N-aminopropylacrylamidehydrochloride, acrylic or methacrylic acid and hydroxyethylmethacrylate.

[0075] Additional monomers that provide the repeating units representedby “Z” in the Structure III above include any useful hydrophilic oroleophilic ethylenically unsaturated polymerizable monomer that mayprovide desired physical or printing properties to the hydrophilicimaging layer. Such monomers include, but are not limited to, acrylates,methacrylates, isoprene, acrylonitrile, styrene and styrene derivatives,acrylamides, methacrylamides, acrylic or methacrylic acid and vinylhalides.

[0076] Representative Class I polymers are identified below as Polymers1 and 3-6. Mixtures of these polymers can also be used. Polymer 2 belowis a precursor to a useful Class I polymer. Further details of thesepolymers and methods of their preparation are provided in U.S. Pat. No.6,190,831 (Leon et al.).

[0077] Polymer 1: Poly (1-vinyl-3-methylimidazoliumchloride-co-N-(3-aminopropyl) methacrylamide hydrochloride),

[0078] Polymer 2: Poly(methyl methacrylate-co-4-vinylpyridine),

[0079] Polymer 3: Poly(methyl methacrylate-co-N-methyl-4-vinylpyridiniumformate),

[0080] Polymer 4: Poly(methyl methacrylate-co-N-butyl-4-vinylpyridiniumformate),

[0081] Polymer 5: Poly(methyl methacrylate-co-2-vinylpyridine), and

[0082] Polymer 6: Poly(methyl methacrylate-co-N-methyl-2-vinylpyridiniumformate).

[0083] Class II Polymers

[0084] The Class II polymers also generally have a molecular weight ofat least 1000. They can be any of a wide variety of vinyl or non-vinylhomopolymers and copolymers.

[0085] Non-vinyl polymers of Class II include, but are not limited to,polyesters, polyamides, polyamide-esters, polyarylene oxides andderivatives thereof, polyurethanes, polyxylylenes and derivativesthereof, silicon-based sol gels (solsesquioxanes), polyamidoamines,polyimides, polysulfones, polysiloxanes, polyethers, poly(etherketones), poly(phenylene sulfide) ionomers, polysulfides andpolybenzimidazoles. Preferably, such non-vinyl polymers are siliconbased sol gels, polyarylene oxides, poly(phenylene sulfide) ionomers orpolyxylylenes, and most preferably, they are poly(phenylene sulfide)ionomers. Procedures and reactants needed to prepare all of these typesof polymers are well known. With the additional teaching providedherein, the known polymer reactants and conditions can be modified by askilled artisan to incorporate or attach a suitable cationic organooniummoiety.

[0086] Silicon-based sol gels useful in this invention can be preparedas a crosslinked polymeric matrix containing a silicon colloid derivedfrom di-, tri- or tetraalkoxy silanes. These colloids are formed bymethods described in U.S. Pat. No. 2,244,325 (Bird), U.S. Pat. No.2,574,902 (Bechtold et al.), and U.S. Pat. No. 2,597,872 (Iler). Stabledispersions of such colloids can be conveniently purchased fromcompanies such as the DuPont Company. A preferred sol-gel usesN-trimethoxysilylpropyl-N,N,N-trimethylammonium acetate both as thecrosslinking agent and as the polymer layer forming material.

[0087] The presence of an organoonium moiety that is chemicallyincorporated into the polymer in some fashion apparently provides orfacilitates the “switching” of the imaging layer from hydrophilic tooleophilic in the exposed areas upon exposure to energy that provides orgenerates heat, when the cationic moiety reacts with its counterion. Thenet result is the loss of charge. Such reactions are more easilyaccomplished when the anion of the organoonium moiety is morenucleophilic and/or more basic, as described above for the Class Ipolymers.

[0088] The organoonium moiety within the polymer can be chosen from atrisubstituted sulfur moiety (organosulfonium), a tetrasubstitutednitrogen moiety (organoammonium), or a tetrasubstituted phosphorousmoiety (organophosphonium). The tetrasubstituted nitrogen(organoammonium) moieties are preferred. This moiety can be chemicallyattached to (that is, pendant) the polymer backbone, or incorporatedwithin the backbone in some fashion, along with the suitable counterion.In either embodiment, the organoonium moiety is present in sufficientrepeating units of the polymer (at least 15 mol %) so that theheat-activated reaction described above can occur to provide desiredhydrophobicity of the imaging layer. When chemically attached as apendant group, the organoonium moiety can be attached along a principalbackbone of the polymer, or to one or more branches of a polymericnetwork, or both. When chemically incorporated within the polymerbackbone, the moiety can be present in either cyclic or acyclic form,and can also form a branching point in a polymer network. Preferably,the organoonium moiety is provided as a pendant group along thepolymeric backbone. Pendant organoonium moieties can be chemicallyattached to the polymer backbone after polymer formation, or functionalgroups on the polymer can be converted to organoonium moieties usingknown chemistry. For example, pendant quaternary ammonium groups can beprovided on a polymeric backbone by the displacement of a “leavinggroup” functionality (such as a halogen) by a tertiary aminenucleophile. Alternatively, the organoonium group can be present on amonomer that is then polymerized or derived by the alkylation of aneutral heteroatom unit (trivalent nitrogen or phosphorous group ordivalent sulfur group) already incorporated within the polymer.

[0089] The organoonium moiety is substituted to provide a positivecharge. Each substituent must have at least one carbon atom that isdirectly attached to the sulfur, nitrogen or phosphorus atom of theorganoonium moiety. Useful substituents include, but are not limited to,substituted or unsubstituted alkyl groups having 1 to 12 carbon atomsand preferably from 1 to 7 carbon atoms (such as methyl, ethyl,n-propyl, isopropyl, t-butyl, hexyl, methoxyethyl, isopropoxymethyl,substituted or unsubstituted aryl groups (phenyl, naphthyl,p-methylphenyl, m-methoxyphenyl, p-chlorophenyl, p-methylthiophenyl,p-N,N-dimethylaminophenyl, xylyl, methoxycarbonylphenyl andcyanophenyl), and substituted or unsubstituted cycloalkyl groups having5 to 8 carbon atoms in the carbocyclic ring (such as cyclopentyl,cyclohexyl, 4-methylcyclohexyl and 3-methylcyclohexyl). Other usefulsubstituents would be readily apparent to one skilled in the art, andany combination of the expressly described substituents is alsocontemplated.

[0090] The organoonium moieties include any suitable anion as describedabove for the Class I polymers. The halides and carboxylates arepreferred.

[0091] In addition, vinyl Class II polymers can be used in the practiceof this invention. Like the non-vinyl polymers, such heat-sensitivepolymers are composed of recurring units having one or more types oforganoonium group. For example, such a polymer can have recurring unitswith both organoammonium groups and organosulfonium groups. It is alsonot necessary that all of the organoonium groups have the same alkylsubstituents. For example, a polymer can have recurring units havingmore than one type of organoammonium group. Useful anions in thesepolymers are the same as those described above for the non-vinylpolymers. In addition, the halides and carboxylates are preferred.

[0092] The organoonium group is present in sufficient recurring units ofthe polymer so that the heat-activated reaction described above canoccur to provide desired hydrophobicity of the imaged printing layer.The group can be attached along a principal backbone of the polymer, orto one or more branches of a polymeric network, or both. Pendant groupscan be chemically attached to the polymer backbone after polymerformation using known chemistry. For example, pendant organoammonium,organophosphonium or organosulfonium groups can be provided on apolymeric backbone by the nucleophilic displacement of a pendant leavinggroup (such as a halide or sulfonate ester) on the polymeric chain by atrivalent amine, divalent sulfur or trivalent phosphorous nucleophile.Pendant onium groups can also be provided by alkylation of correspondingpendant neutral heteroatom groups (nitrogen, sulfur or phosphorous)using any commonly used alkylating agent such as alkyl sulfonate estersor alkyl halides. Alternatively a monomer precursor containing thedesired organoammonium, organophosphonium or organosulfonium group maybe polymerized to yield the desired polymer.

[0093] The organoammonium, organophosphonium or organosulfonium group inthe vinyl polymer provides the desired positive charge. Generally,preferred pendant organoonium groups can be illustrated by the followingStructures IV, V, and VI:

[0094] wherein R is a substituted or unsubstituted alkylene group having1 to 12 carbon atoms that can also include one or more oxy, thio,carbonyl, amido, or alkoxycarbonyl groups with the chain (such asmethylene, ethylene, isopropylene, methylenephenylene,methyleneoxymethylene, n-butylene, and hexylene), a substituted orunsubstituted arylene group having 6 to 10 carbon atoms in the ring(such as phenylene, naphthylene, xylylene, and 3-methoxyphenylene), or asubstituted or unsubstituted cycloalkylene group having 5 to 10 carbonatoms in the ring (such as 1,4-cyclohexylene and3-methyl-1,4-cyclohexylene). In addition, R can be a combination of twoor more of the defined substituted or unsubstituted alkylene, arylene,and cycloalkylene groups. Preferably, R is a substituted orunsubstituted ethyleneoxycarbonyl or phenylenemethylene group. Otheruseful substituents not listed herein could include combinations of anyof those groups listed above as would be readily apparent to one skilledin the art.

[0095] R₃, R₄ and R₅ are independently substituted or unsubstitutedalkyl group having 1 to 12 carbon atoms (such as methyl, ethyl,n-propyl, isopropyl, t-butyl, hexyl, hydroxymethyl, methoxymethyl,benzyl, methylenecarboalkoxy, and cyanoalkyl), a substituted orunsubstituted aryl group having 6 to 10 carbon atoms in the carbocyclicring (such as phenyl, naphthyl, xylyl, p-methoxyphenyl, p-methylphenyl,m-methoxyphenyl, p-chlorophenyl, p-methylthiophenyl,p-N,N-dimethylaminophenyl, methoxycarbonylphenyl, and cyanophenyl), or asubstituted or unsubstituted cycloalkyl group having 5 to 10 carbonatoms in the carbocyclic ring (such as 1,3- or 1,4-cyclohexyl).Alternatively, any two of R₃, R₄ and R₅ can be combined to form asubstituted or unsubstituted heterocyclic ring with the chargedphosphorus, sulfur or nitrogen atom, the ring having 4 to 8 carbon,nitrogen, phosphorus, sulfur or oxygen atoms in the ring. Suchheterocyclic rings include, but are not limited to, substituted orunsubstituted morpholinium, piperidinium, and pyrrolidinium groups forStructure VI. Other useful substituents for these various groups wouldbe readily apparent to one skilled in the art, and any combinations ofthe expressly described substituents are also contemplated.

[0096] Preferably, R₃, R₄, and R₅ are independently substituted orunsubstituted methyl or ethyl groups.

[0097] W⁻ is any suitable anion as described above for the Class Ipolymers. Acetate and chloride are preferred anions.

[0098] Polymers containing quaternary ammonium groups as describedherein are most preferred vinyl Class II polymers.

[0099] The vinyl Class II polymers useful in the practice of thisinvention can be represented by the following Structure VII thatrepresents random recurring units derived from one or more monomers asdescribed below in Structure VII:

[0100] wherein X′ represents recurring units to which the organooniumgroups (“ORG”) are attached, Y′ represents recurring units derived fromethylenically unsaturated polymerizable monomers that may provide activesites for crosslinking using any of various crosslinking mechanisms(described below), and Z′ represents recurring units derived from anyadditional ethylenically unsaturated polymerizable monomers. The variousrecurring units are present in suitable amounts, as represented by x′being from about 15 to about 99 mol %, y′ being from about 1 to about 20mol %, and z′ being from 0 to about 84 mol %. Preferably, x′ is fromabout 20 to about 98 mol %, y′ is from about 2 to about 10 mol %, and z′is from 0 to about 78 mol %. W⁻ is a suitable cation as described above.

[0101] Crosslinking of the vinyl polymer can be achieved in the same wayas described above for the Class I polymers.

[0102] Additional monomers that provide the additional recurring unitsrepresented by Z′ in Structure VII include any useful hydrophilic oroleophilic ethylenically unsaturated polymerizable monomer that mayprovide desired physical or printing properties to the imaging layer.Such monomers include, but are not limited to, acrylates, methacrylates,acrylonitrile, isoprene, styrene and styrene derivatives, acrylamides,methacrylamides, acrylic or methacrylic acid, and vinyl halides.

[0103] Representative Class II non-vinyl polymers are identified hereinbelow as Polymers 7-8 and 10-18. Mixtures of these polymers can also beused. Polymer 9 is a precursor to Polymer 10. Further details of suchpolymers and methods of preparing them are provided in U.S. Pat. No.6,109,830 (Leon et al).

[0104] Polymer 7: Poly(p-xylidenetetrahydro-thiophenium chloride),

[0105] Polymer 8: Poly[phenylenesulfide-co-methyl(4-thiophenyl)sulfonium chloride],

[0106] Polymer 9: Brominated poly(2,6-dimethyl-1,4-phenylene oxide),

[0107] Polymer 10: Dimethyl sulfonium bromide derivative ofpoly(2,6-dimethyl-1,4-phenylene oxide),

[0108] Polymer 11: Poly[methyl methacrylate-co-2-trimethylammoniumethylmethacrylic chloride-co-N-(3-aminopropyl) methacrylamide hydrochloride],

[0109] Polymer 12: Poly[methyl methacrylate-co-2-trimethylammoniumethylmethacrylic acetate-co-N-(3-aminopropyl) methacrylamide],

[0110] Polymer 13: Poly[methyl methacrylate-co-2-trimethylammoniumethylmethacrylic fluoride-co-N-(3-aminopropyl) methacrylamide hydrochloride],

[0111] Polymer 14: Poly[vinylbenzyl trimethylammoniumchloride-co-N-(3-aminopropyl) methacrylamide hydrochloride],

[0112] Polymer 15: Poly([vinylbenzyltrimethyl-phosphoniumacetate-co-N-(3-aminopropyl) methacrylamide hydrochloride],

[0113] Polymer 16: Poly [dimethyl-2-(methacryloyloxy) ethylsulfoniumchloride-co-N-(3-aminopropyl) methacrylamide hydrochloride],

[0114] Polymer 17: Poly [vinylbenzyldimethylsulfonium methylsulfate],and

[0115] Polymer 18: Poly[vinylbenzyldimethylsulfonium chloride].

[0116] Class III Polymers

[0117] Each of the Class III polymers has a molecular weight of at least1000, and preferably of at least 5000. For example, the polymers can bevinyl homopolymers or copolymers prepared from one or more ethylenicallyunsaturated polymerizable monomers that are reacted together using knownpolymerization techniques and reactants. Alternatively, they can beaddition homopolymers or copolymers (such as polyethers) prepared fromone or more heterocyclic monomers that are reacted together using knownpolymerization techniques and reactants. Additionally, they can becondensation type polymers (such as polyesters, polyimides, polyamidesor polyurethanes) prepared using known polymerization techniques andreactants. Whatever the type of polymers, at least 15 mol % (preferably20 mol %) of the total recurring units in the polymer comprise thenecessary heat-activatable thiosulfate groups.

[0118] The Class III polymers useful in the practice of this inventioncan be represented by the following Structure VIII wherein thethiosulfate group (or Bunte salt) is a pendant group:

[0119] wherein A represents a polymeric backbone, R₆ is a divalentlinking group, and Y₁ is hydrogen or a cation.

[0120] Useful polymeric backbones include, but are not limited to, vinylpolymers, polyethers, polyimides, polyamides, polyurethanes andpolyesters. Preferably, the polymeric backbone is a vinyl polymer orpolyether.

[0121] Useful R₆ linking groups include —(COO)_(n1)(Z₁)_(m)— wherein n1is 0 or 1, m is 0 or 1, and Z₁ is a substituted or unsubstitutedalkylene group having 1 to 6 carbon atoms (such as methylene, ethylene,n-propylene, isopropylene, butylenes, 2-hydroxypropylene, and2-hydroxy-4-azahexylene) that can have one or more oxygen, nitrogen orsulfur atoms in the chain, a substituted or unsubstituted arylene grouphaving 6 to 14 carbon atoms in the aromatic ring (such as phenylene,naphthalene, anthracylene and xylylene), or a substituted orunsubstituted arylenealkylene (or alkylenearylene) group having 7 to 20carbon atoms in the chain (such as p-methylenephenylene,phenylenemethylene-phenylene, biphenylene, andphenyleneisopropylenephenylene). In addition, R₆ can be an alkylenegroup, an arylene group, in an arylenealkylene group as defined abovefor Z₁.

[0122] Preferably, R₆, is a substituted or unsubstituted of alkylenegroup of 1 to 3 carbon atoms, a substituted or unsubstituted arylenegroup of 6 carbon atoms in the aromatic ring, an arylenealkylene groupof 7 or 8 carbon atoms in the chain, or —COO(Z₁)_(m)— wherein Z₁ ismethylene, ethylene or phenylene. Most preferably, R₆ is phenylene,methylene or —COO—.

[0123] Y₁ is hydrogen, ammonium ion, or a metal ion (such as sodium,potassium, magnesium, calcium, cesium, barium, zinc, or lithium ion).Preferably, Y₁ is hydrogen, ammonium, sodium, or potassium ion.

[0124] As the thiosulfate group is generally pendant to the backbone,preferably it is part of an ethylenically unsaturated polymerizablemonomer that can be polymerized using conventional techniques to formvinyl homopolymers of the thiosulfate-containing recurring units, orvinyl copolymers when copolymerized with one or more additionalethylenically unsaturated polymerizable monomers. Thethiosulfate-containing recurring units generally comprise at least 15mol % of all recurring units in the polymer, preferably they comprisefrom about 20 to 100 mol % of all recurring units. A polymer can includemore than one type of repeating unit containing a thiosulfate group asdescribed herein.

[0125] Polymers having the above-described thiosulfate group arebelieved to crosslink and to switch from hydrophilic thiosulfate tohydrophobic disulfide (upon loss of sulfate) with heating.

[0126] Thiosulfate-containing molecules (or Bunte salts) can be preparedfrom the reaction between an alkyl halide and thiosulfate salt as taughtby Bunte, Chem.Ber. 7, 646, 1884. Polymers containing thiosulfate groupscan either be prepared from functional monomers or from preformedpolymers. Polymers can also be prepared from preformed polymers in asimilar manner as described in U.S. Pat. No. 3,706,706 (Vandenberg).Thiosulfate-containing molecules can also be prepared by reaction of analkyl epoxide with a thiosulfate salt, or between an alkyl epoxide and amolecule containing a thiosulfate moiety (such as2-aminoethanethiosulfuric acid), and the reaction can be performedeither on a monomer or polymer as illustrated by Thames, Surf. Coating,3 (Waterborne Coat.), Chapter 3, pp. 125-153, Wilson et al (Eds.).

[0127] Details for making Class III polymers are provided in U.S. Pat.No. 5,985,514 (noted above), incorporated herein by reference.

[0128] Vinyl polymers can be prepared by copolymerizing monomerscontaining the thiosulfate functional groups with one or more otherethylenically unsaturated polymerizable monomers to modify polymerchemical or functional properties, to optimize imaging memberperformance, or to introduce additional crosslinking capability.

[0129] Useful additional ethylenically unsaturated polymerizablemonomers include, but are not limited to, acrylates (includingmethacrylates) such as ethyl acrylate, n-butyl acrylate, methylmethacrylate and t-butyl methacrylate, acrylamides (includingmethacrylamides), an acrylonitrile (including methacrylonitrile), vinylethers, styrenes, vinyl acetate, dienes (such as ethylene, propylene,1,3-butadiene, and isobutylene), vinyl pyridine and vinylpyrrolidone.Acrylamides, acrylates, and styrenes are preferred.

[0130] Useful polymers of Class III include, for example:

[0131] Polymer 19: Poly(chloromethyl-ethylene oxide-co-sodiumthiosulfate methyl-ethylene oxide),

[0132] Polymer 20: Poly(vinyl benzyl thiosulfate sodium salt-co-methylmethacrylate),

[0133] Polymer 21: Poly[vinyl benzyl thiosulfate sodiumsalt-co-N-(3-aminopropyl)methacylamide hydrochloride],

[0134] Polymer 22: Poly(vinyl benzyl thiosulfate sodium salt),

[0135] Polymer 23: Poly(2-sodium thiosulfate-co-ethyl methacrylate),

[0136] Polymer 24: Poly[2-hydroxy-3-sodium thiosulfate-propylmethacrylate-co-2-(methacryloyloxy)ethyl acetoacetate), and

[0137] Polymer 25: Poly(4-aza-2-hydroxy-6-sodium thiosulfate-hexylmethacrylate).

[0138] The imaging layer of the imaging member can include one or moreionomers with or without minor amounts (less than 20 weight %, based ontotal dry weight of the layer) of additional binder or polymericmaterials that will not adversely affect its imaging properties.

[0139] In the composition used to provide the heat-sensitive layer, theamount of ionomer is generally present in an amount of at least 1 weight%, and preferably at least 2 weight %. A practical upper limit of theamount of ionomer in the composition is about 10 weight %.

[0140] The amount of ionomer used in the imaging layer is generally atleast 0.1 g/m², and preferably from about 0.1 to about 10 g/m² (dryweight). This generally provides an average dry layer thickness of fromabout 0.1 to about 10 μm.

[0141] The imaging layer can also include one or more conventionalsurfactants for coatability or other properties, dyes or colorants toallow visualization of the written image, or any other addenda commonlyused in the lithographic art, as long as the concentrations are lowenough so they are inert with respect to imaging or printing properties.

[0142] It is essential that the heat-sensitive imaging layer includesone or more photothermal conversion materials to absorb appropriateradiation from an appropriate energy source (such as a laser), whichradiation is converted into heat. Thus, such materials convert photonsinto heat. Preferably, the radiation absorbed is in the infrared andnear-infrared regions of the electromagnetic spectrum. At least one ofthe photothermal conversion materials used in this invention is anegatively-charged oxonol IR dye that comprise a methine linkageconjugated to a negatively-charged group.

[0143] It is also preferred that the negatively-charged oxonol IR dye besoluble in water or any of the water-miscible organic solvents that aredescribed below as useful for preparing heat-sensitive compositions.More preferably, these IR dyes are soluble in either water or methanol,or a mixture of water and methanol. Solubility in water or thewater-miscible organic solvents means that the negatively-charged oxonolIR dye can be dissolved at a concentration of at least 0.5 g/l at roomtemperature at room temperature.

[0144] The negatively-charged oxonol IR dyes are sensitive to radiationin the near-infrared and infrared regions of the electromagneticspectrum. Thus, they generally have a λ_(max) at or above 700 nm(preferably a λ_(max) of from about 750 to about 900 nm, and morepreferably a λ_(max) of from about 800 to about 850 nm).

[0145] The negatively-charged oxonol IR dyes useful in this inventionare generally anionic dyes having a polymethine chain conjugated with 2cyclic or aliphatic groups, one of which is negatively charged.

[0146] Useful negatively-charged oxonol IR dyes can be synthesized usinggeneral procedures described by Hamer in The Cyanine Dyes and RelatedCompounds, Interscience Publishers, 1964. A preferred synthetic methodis described below. The dyes may be provided for incorporation into theheat-sensitive formulations of this invention in any suitable manner. Ina preferred embodiment, the dyes are dissolved in a suitable organicsolvent.

[0147] Useful negatively-charged oxonol IR dyes useful in the practiceof this invention can be represented by the following Structure I:

[0148] wherein R′ is a substituted or unsubstituted alkyl group having 1to 20 carbon atoms (methyl, ethyl, isopropyl, t-butyl, hexyl, dodecyl,aminoethyl, methylsulfonaminoethyl, and other groups readily apparent toone skilled in the art), substituted or unsubstituted carbocyclicaromatic groups (such as phenyl, naphthyl, xylyl, m-carboxyphenyl, andothers than would be readily apparent to one skilled in the art),substituted or unsubstituted heterocyclic groups (aromatic ornon-aromatic) having 3 to 8 carbon, oxygen, nitrogen and sulfur atoms inthe ring structure (such as morpholino, pyridyl, pyrimidyl,thiomorpholino, pyrrolidinyl, piperazinyl, and others that would bereadily apparent to one skilled in the art), or a substituted orunsubstituted cycloalkyl group having 4 to 12 carbon atoms in the ringsystem including fused ring systems (such as cyclopenyl, cyclohexyl, andothers that would be readily apparent to one skilled in the art).

[0149] Preferably, R′ is a substituted or unsubstituted alkyl grouphaving 1 to 10 carbon atoms or a substituted or unsubstituted phenylgroup. More preferably, R′ is a substituted or unsubstituted alkyl grouphaving 1 to 4 carbon atoms (such as substituted or unsubstituted methyl,ethyl, n-propyl, iso-propyl, and t-butyl groups) or a substituted orunsubstituted phenyl group. Most preferably, R′ is an unsubstitutedmethyl, ethyl, isopropyl, or phenyl group.

[0150] R₁′ and R₂′ are independently substituted or unsubstitutedheterocyclic or carbocyclic aromatic groups having from 5 to 12 atoms inthe aromatic ring (including fused ring systems). Preferably, R₁′ andR₂′ represent the same aromatic group. Useful aromatic groups include,but are not limited to, substituted or unsubstituted phenyl groups,substituted or unsubstituted naphthyl groups, substituted orunsubstituted furyl groups, substituted and unsubstituted thiophenylgroups, and substituted or unsubstituted benzofuryl groups. Thesearomatic groups can be substituted with one or more amino, methoxy,carboxy, sulfo, sulfonamido, or alkylsulfonyl groups. Preferably, whenR₁′ and R₂′ are substituted, they each have one or more of the samesubstituents.

[0151] M⁺ is a suitable monovalent cation such as an alkali metal ion(lithium, sodium or potassium), an ammonium ion, a trialkylammonium ion(such as trimethylammonium, triethyleammonium or tributylammonium ions),a tetraalkylammonium ion (such as tetramethylammonium ion), pyridiniumion, or tetramethyl guanidinium ion.

[0152] A preferred class of compounds of this invention are thoserepresented by the Structure I shown above wherein R′ is a substitutedor unsubstituted alkyl group having 1 to 4 carbon atoms or substitutedor unsubstituted carbocyclic aryl group (such as a phenyl group), andR₁′ and R₂′ are independently substituted or unsubstituted carbocyclicaromatic groups (that is aryl groups such as phenyl groups).

[0153] The dyes of this invention can exist in several tautomeric forms,for example as shown below:

[0154] Examples of oxonol IR dyes of this invention include, but are notlimited to, the following compounds:

[0155] The one or more negatively-charged oxonol IR dyes are present inthe heat-sensitive or thermal imaging composition of this invention inan amount of generally at least 0.2 weight % (% solids), and preferablyat least 0.4 weight %. The upper limit of IR dye is not critical but isgoverned by the IR dye cost, desired thermal sensitivity and solventsolubility. A practical limit may be about 1 weight %. The amount of IRdye is provided in the heat-imaging layer of an imaging membersufficient to provide a transmission optical density of at least 0.1,and preferably of at least 0.3 when exposed to radiation having aλ_(max) of 830 nm.

[0156] The heat-sensitive compositions and imaging layers can includeadditional photothermal conversion materials, although the presence ofsuch materials is not preferred. Such optional materials can be other IRdyes, carbon black, polymer-grafted carbon, IR-absorbing pigments,evaporated pigments, semiconductor materials, alloys, metals, metaloxides, metal sulfides or combinations thereof, or a dichroic stack ofmaterials that absorb radiation by virtue of their refractive index andthickness. Borides, carbides, nitrides, carbonitrides, bronze-structuredoxides and oxides structurally related to the bronze family but lackingthe WO₂₉ component, are also useful. Useful absorbing dyes for nearinfrared diode laser beams are described, for example, in U.S. Pat. No.4,973,572 (DeBoer). Particular dyes of interest are “broad band” dyes,that is those that absorb over a wide band of the spectrum.

[0157] Alternatively, the same or different photothermal conversionmaterial (including a negatively-charged oxonol IR dye described herein)can be provided in a separate layer that is in thermal contact with theheat-sensitive imaging layer. Thus, during imaging, the action of theadditional photothermal conversion material can be transferred to theheat-sensitive imaging layer.

[0158] The heat-sensitive composition of this invention can be appliedto a support using any suitable equipment and procedure, such as spincoating, knife coating, gravure coating, dip coating or extrusion hoppercoating. In addition, the composition can be sprayed onto a support,including a cylindrical support, using any suitable spraying means forexample as described in U.S. Pat. No. 5,713,287 (noted above).

[0159] The heat-sensitive compositions of this invention are generallyformulated in and coated from water or water-miscible organic solventsincluding, but not limited to, water-miscible alcohols (for example,methanol, ethanol, isopropanol, 1-methoxy-2-propanol, and n-propanol),methyl ethyl ketone, tetrahydrofuran, acetonitrile,N-N-dimethylformamide, butyrolactone, and acetone. Water, methanol,ethanol, and 1-methoxy-2-propanol are preferred. Mixtures (such as amixture of water and methanol) of these solvents can also be used ifdesired. By “water-miscible” is meant that the solvent is soluble inwater at all proportions at room temperature.

[0160] While the heat-sensitive compositions of this invention arepreferably used in the lithographic printing plates described herein,they can be used for various other situations where a heat-sensitivecomposition may be useful to provide images.

[0161] The imaging members of this invention can be of any useful formincluding, but not limited to, printing plates, printing cylinders,printing sleeves, and printing tapes (including flexible printing webs),all of any suitable size or dimensions. Preferably, the imaging membersare printing plates or on-press cylinders. Imaging members can alsoinclude elements that are not necessarily used in lithographic imagingand printing, but that are useful in other imaging systems.

[0162] During use, the imaging member of this invention is exposed to asuitable source of energy that generates or provides heat, such as afocused laser beam or a thermoresistive head, in the foreground areaswhere ink is desired in the printed image, typically from digitalinformation supplied to the imaging device. A laser used to expose theimaging members of this invention is preferably a diode laser, becauseof the reliability and low maintenance of diode laser systems, but otherlasers such as gas or solid state lasers may also be used. Thecombination of power, intensity and exposure time for laser imagingwould be readily apparent to one skilled in the art. Specifications forlasers that emit in the near-IR region, and suitable imagingconfigurations and devices are described in U.S. Pat. No. 5,339,737(Lewis et al), incorporated herein by reference with respect to suchimaging devices. The imaging member is typically sensitized so as tomaximize responsiveness at the emitting wavelength of the laser.

[0163] The imaging apparatus can operate on its own, functioning solelyas a platemaker, or it can be incorporated directly into a lithographicprinting press. In the latter case, printing may commence immediatelyafter imaging, thereby reducing press set-up time considerably. Theimaging apparatus can be configured as a flatbed recorder or as a drumrecorder, with the imaging member mounted to the interior or exteriorcylindrical surface of the drum.

[0164] In the drum configuration, the requisite relative motion betweenan imaging device (such as laser beam) and the imaging member can beachieved by rotating the drum (and the imaging member mounted thereon)about its axis, and moving the imaging device parallel to the rotationaxis, thereby scanning the imaging member circumferentially so the image“grows” in the axial direction. Alternatively, the beam can be movedparallel to the drum axis and, after each pass across the imagingmember, incremented angularly so that the image “grows”circumferentially. In both cases, after a complete scan by the laserbeam, an image corresponding to the original document or picture can beapplied to the surface of the imaging member.

[0165] In the flatbed configuration, a laser beam is drawn across eitheraxis of the imaging member, and is indexed along the other axis aftereach pass. Obviously, the requisite relative motion can be produced bymoving the imaging member rather than the laser beam.

[0166] While laser imaging is preferred in the practice of thisinvention, imaging can be provided by any other means that provides orgenerates thermal energy in an imagewise fashion. For example, imagingcan be accomplished using a thermoresistive head (thermal printing head)in what is known as “thermal printing”, described for example in U.S.Pat. No. 5,488,025 (Martin et al). Such thermal printing heads arecommercially available (for example, as Fujisu Thermal Head FTP-040MCS001 and TDK Thermal Head F415 HH7-1089).

[0167] Imaging of heat-sensitive compositions on printing presscylinders can be accomplished using any suitable means, for example, astaught in U.S. Pat. No. 5,713,287 (noted above).

[0168] After imaging, the imaging member can be used for printingwithout conventional wet processing. Applied ink can be imagewisetransferred to a suitable receiving material (such as cloth, paper,metal, glass, or plastic) to provide one or more desired impressions. Ifdesired, an intermediate blanket roller can be used to transfer the inkfrom the imaging member to the receiving material. The imaging memberscan be cleaned between impressions, if desired, using conventionalcleaning means.

[0169] The following examples illustrate the practice of the invention,and are not meant to limit it in any way. The synthetic methods arepresented to show how some of the preferred heat-sensitive polymers andnegatively charged oxonol IR dyes can be prepared.

[0170] Synthesis of IR Dyes:

[0171] Oxonol IR Dye 1 was prepared using the following synthetic schemethat is generally useful for all of the compounds of this invention.

[0172] A sample of the noted cyano compound (6.4 g, 0.02 mole) washeated with 0.5 mole equivalents of sarcosine (commercially availablefrom Aldrich Chemical Co.) in acetic anhydride to boiling. The reactionsolution was heated for 5 minutes and triethylamine (5 ml) was added.The solution turned dark blue and after another 5 minutes a green solidprecipitated. The solid was collected by filtration and washed 3 timeswith CH₃CN. The solid was dried 16 hours in a vacuum oven at 40° C. Thestructure was shown to be consistent with IR Dye by NMR and wasdetermined to be >95% pure by HPLC (λ_(max) 786 nm (CH₃OH), λ_(max)12.4×10⁴).

[0173] IR Dye 2 was similarly prepared and identified except thatpyridine was used in place of triethylamine and phenyl-NHCH₂COOH wasused in place of sarcosine.

[0174] The following examples illustrate the practice of this inventionand its advantages over embodiments outside of the scope of theinvention. The invention is not to be construed as limited to theseexamples.

INVENTION EXAMPLE 1 AND COMPARATIVE EXAMPLE 1

[0175] Imaging formulations 1 and 2 were prepared using the components(parts by weight) shown in TABLE I below. TABLE I Formulation 1Formulation 2 Component (Comparative Example 1) (Invention Example 1)Polymer 22 0.30 0.33 IR Dye A 0.033 — Oxonol IR Dye 1 — 0.033 Water 4.143.24 Methanol 4.50 0.90 Acetone — 4.50

[0176] Each formulation was coated at a dry coating weight of about 1.0g/m² onto a grained phosphoric acid-anodized aluminum support. Theresulting printing plates were air-dried. Each imaging layer of theprinting plate was imaged at 830 nm on a plate setter like thecommercially available CREO TRENDSETTER™ (but smaller in size) usingdoses ranging from 360 to 820 mJ/cm².

[0177] The imaging layer in Comparative Example 1 printing plate rapidlydiscolored to a tan color in the exposed regions producing anunmistakable sulfur odor during and after many hours following imaging.By contrast, the blue imaging layer in the Example 1 printing plateproduced a deeper blue image and the undesirable sulfur smell wassignificantly reduced. Thus, the printing plates of this invention werefound to exhibit reduced gaseous effluents upon imaging.

[0178] The imaged Example 1 plate was mounted on the plate cylinder of acommercially available full-page printing press (A. B. Dick 9870duplicator) for press runs. A commercial black ink and Vam UniversalPink fountain solution (from Vam Products Co.) were used. The plate wasdeveloped on press within 60 seconds of the press run and printed withfull density and high image quality for at least 1,000 impressions.

[0179] The invention has been described in detail with particularreference to preferred embodiments thereof, but it will be understoodthat variations and modifications can be effected within the spirit andscope of the invention.

We claim:
 1. A heat-sensitive composition comprising: a) a hydrophilicheat-sensitive charged ionomer, b) water or a water-miscible organicsolvent, and c) an infrared radiation sensitive negatively-chargedoxonol dye (IR dye) that has a λ_(max) greater than 700 nm as measuredin water or a water-miscible organic solvent, said IR dye beingrepresented by the following Structure I:

wherein R′ is a substituted or unsubstituted alkyl group, substituted orunsubstituted cycloalkyl group, or substituted or unsubstitutedheterocyclic group, or a substituted or unsubstituted carbocyclicaromatic group, R′₁ and R′₂ are independently substituted orunsubstituted heterocyclic or carbocyclic aromatic groups, and M⁺ is amonovalent cation.
 2. The heat-sensitive composition of claim 1 whereinsaid IR dye has a λ_(max) of from about 750 to about 900 nm as measuredin water or a water-miscible organic solvent.
 3. The heat-sensitivecomposition of claim 3 wherein said IR dye has a λ_(max) of from about800 to about 850 nm as measured in water or a water-miscible organicsolvent.
 4. The compound of claim 1 wherein R′ is a substituted orunsubstituted alkyl group or a substituted or unsubstituted phenylgroup, and R₁′ and R₂′ are the same substituted or unsubstitutedheterocyclic or carbocyclic aromatic group.
 5. The compound of claim 1wherein R′ is a substituted or unsubstituted alkyl group having 1 to 4carbon atoms, and R₁′ and R₂′ are the same or different substituted orunsubstituted carbocyclic aromatic group.
 6. The compound of claim 5wherein R′ is a substituted or unsubstituted methyl, ethyl, isopropyl,or phenyl group.
 7. The compound of claim 1 wherein R′ is a substitutedor unsubstituted alkyl group having 1 to 4 carbon atoms or a substitutedor unsubstituted phenyl group, and R₁′ and R₂′ are the same substitutedor unsubstituted phenyl group.
 8. The heat-sensitive composition ofclaim 1 wherein said IR dye is one of following compounds:


9. The heat-sensitive composition of claim 1 comprising water, methanol,ethanol, 1-methoxy-2-propanol, acetone, methyl ethyl ketone,acetonitrile, tetrahydrofuran, N-N-dimethylformaminde, butyrolactone, ora mixture of two or more of these solvents.
 10. The heat-sensitivecomposition of claim 1 wherein the heat-sensitive ionomer is selectedfrom the following three classes of polymers: I) a crosslinked oruncrosslinked vinyl polymer comprising recurring units comprisingpositively-charged, pendant N-alkylated aromatic heterocyclic groups,II) a crosslinked or uncrosslinked polymer comprising recurringorganoonium groups, and III) a polymer comprising a pendant thiosulfategroup.
 11. The heat-sensitive composition of claim 10 wherein saidheat-sensitive ionomer is a Class I polymer represented by the followingStructure II:

wherein R₁ is an alkyl group, R₂ is an alkyl group, an alkoxy group, anaryl group, an alkenyl group, halo, a cycloalkyl group, or aheterocyclic group having 5 to 8 atoms in the ring, Z″ represents thecarbon and nitrogen, oxygen, or sulfur atoms necessary to complete anaromatic N-heterocyclic ring having 5 to 10 atoms in the ring, n is 0 to6, and W⁻ is an anion.
 12. The heat-sensitive composition of claim 10wherein said heat-sensitive ionomer is a Class I polymer represented bythe Structure II:

wherein HET⁺ represents a positively-charged, pendant N-alkylatedaromatic heterocyclic group, X represents recurring units havingattached HET⁺ groups, Y represents recurring units derived fromethylenically unsaturated polymerizable monomers that provide activecrosslinking sites, Z represents recurring units for additionalethylenically unsaturated monomers, x is from about 20 to 100 mol %, yis from 0 to about 20 mol %, z is from 0 to about 80 mol %, and W⁻ is ananion.
 13. The heat-sensitive composition of claim 10 wherein saidheat-sensitive ionomer is a Class II vinyl polymer represented by any ofStructures IV, V, and VI:

wherein R is an alkylene, arylene, or cycloalkylene group or acombination of two or more such groups, R₃, R₄ and R₅ are independentlysubstituted or unsubstituted alkyl, aryl or cycloalkyl groups, or anytwo of R₃, R₄ and R₅ can be combined to form a heterocyclic ring withthe charged phosphorus, nitrogen or sulfur atom, and W⁻ is an anion. 14.The heat-sensitive composition of claim 10 wherein said Class II polymeris represented by the following Structure VII:

wherein ORG represents organoonium groups, X′ represents recurring unitsto which the ORG groups are attached, Y′ represents recurring unitsderived from ethylenically unsaturated polymerizable monomers that mayprovide active sites for crosslinking, Z′ represents recurring unitsderived from any additional ethylenically unsaturated polymerizablemonomers, W⁻ is an anion, x′ is from about 20 to about 99 mol %, y′ isfrom about 1 to about 20 mol %, and z′ is from 0 to about 79 mol %. 15.The heat-sensitive composition of claim 10 wherein said heat-sensitiveionomer is a Class III polymer having the following Structure VIII:

wherein A represents a polymeric backbone, R₆ is a divalent linkinggroup, and Y₁ is a hydrogen or a cation.
 16. The heat-sensitivecomposition of claim 15 wherein R₆ is an alkylene group, an arylenegroup, an arylenealkylene group, or —(COO)_(n1)(Z₁)_(m) wherein n1 is 0or 1, and Z₁ is an alkylene group, an arylene group, or anarylenealkylene group, and Y₁ is hydrogen, ammonium ion or a metal ion.17. The heat-sensitive composition of claim 15 wherein R₆ is an alkylenegroup of 1 to 3 carbon atoms, an arylene group of 6 carbon atoms in thearomatic ring, an arylene alkylene group of 7 or 8 carbon atoms in thechain, or —COO(Z₁)_(m) wherein Z₁ is methylene, ethylene or phenylene.18. The heat-sensitive composition of claim 1 wherein saidheat-sensitive ionomer comprises anionic groups within at least 15 mol %of the polymer recurring units.
 19. The heat-sensitive composition ofclaim 1 wherein said heat-sensitive ionomer is present at from about 1to about 10 weight %, and said IR dye is present at from about 0.2 toabout 1 weight %.
 20. A negative-working imaging member comprising asupport having disposed thereon a hydrophilic imaging layer preparedfrom the heat-sensitive composition of claim
 1. 21. The imaging memberof claim 20 wherein said heat-sensitive ionomer is present in saidimaging layer in an amount of at least 0.1 g/m², and said IR dye ispresent in said imaging layer in an amount sufficient to provide atransmission optical density of at least 0.1 when exposed to radiationhaving a λ_(max) of 830 nm.
 22. The imaging member of claim 20 whereinsaid support is an on-press printing cylinder.
 23. A method of imagingcomprising the steps of: A) providing the negatively-working imagingmember of claim 23, and B) imagewise exposing said imaging member toprovide exposed and unexposed areas in the imaging layer of said imagingmember, whereby said exposed areas are rendered more hydrophobic thansaid unexposed areas by heat provided by said imagewise exposure. 24.The method of claim 23 wherein said imagewise exposing is carried outusing an IR radiation emitting laser, and said imaging member is alithographic printing plate having an aluminum support or an imagingcylinder.
 25. The method of claim 23 wherein said imagewise exposing isaccomplished using a thermal head.
 26. A method of printing comprisingthe steps of: A) providing the negative-working imaging member of claim23, B) imagewise exposing said imaging member to provide exposed andunexposed areas in the imaging layer of said imaging member, wherebysaid exposed areas are rendered more hydrophobic than said unexposedareas by heat provided by said imagewise exposure, and C) contactingsaid imagewise exposed imaging member with a lithographic printing ink,and imagewise transferring said printing ink from said imaging member toa receiving material.
 27. A method of imaging comprising the steps of:A) spray coating the heat-sensitive composition of claim 1 onto asupport to provide a negative-working imaging member, and B) imagewiseexposing said imaging member to provide exposed and unexposed areas inthe imaging layer of said imaging member, whereby said exposed areas arerendered more hydrophobic than said unexposed areas by heat provided bysaid imagewise exposure.
 28. The method of claim 27 wherein said supportis an on-press printing cylinder or sleeve.